The elements that shape a propagating wavefront are a key part of any optical system. Diffractive optical elements offer major advantages over conventional refractive optical elements in terms of size, weight, and cost. Bulky groups of classical optical elements, such as lenses, mirrors, beam splitters and filters, are replaced by a single diffractive optical element. As a result, optical systems can be made smaller, more robust and less expensive. In addition, these devices can perform complex waveshaping and wavelength dispersing functions that are often beyond the capabilities of conventional elements.
A diffractive optical element includes a pattern of structures which can modulate and transform light in a predetermined way. The element utilizes precision surfaces that have a series of grooves, which have small steps at the groove boundaries. The placement of these grooves allows an optical designer to precisely shape the emerging optical wavefront. The required step heights at the groove boundaries are typically between 1 and 10 μm. A scoring tool with very high resolution and large flexibility is needed to manufacture these micro-structures having arbitrary shapes.
As is widely known, convex and concave diffraction-gratings are useful in the field of spectroscopy. Concave diffraction gratings are used as a stand-alone device for generating either a Roland Circle spectrometer or a spectrometer that has aberration correction characteristics that are caused by using a variable groove spacing across the surface of the part. Convex diffraction gratings are useful in systems that use an Offner spectrometer design. Instead of a diamond-turning technique such as described above, the manufacture of these gratings is often done through either the use of holographic techniques to obtain the desired groove spacings with a near sinusoidal groove profile, or a holographic technique for generating the groove spacing and an ion etching process to create a blazed groove profile. There are also techniques for generating concave-ruled diffraction gratings that involve elaborate mechanical geometries. Ruled diffraction gratings are generally manufactured by using a single (non-turning) diamond 3 (see FIG. 1 which illustrates a prior art convex diffraction grating 1) that has an angled tip 4. The diamond tip is dragged across the substrate having a convex surface 2 and creates a single groove 5 with each pass of the diamond 3. This works sufficiently for planar gratings where the groove angle does not change across the grating. In this case, the diamond can easily be drawn across the planar grating surface while maintaining the same angle between the diamond tip and the substrate. However, in the case of a non-planar grating, a precise mechanical system is required to maintain the angle between the diamond tip and the non-planar substrate.
Thus, it is desirable to provide a non-planar optical diffraction grating which is able to overcome the above disadvantages and which can be easily manufactured in an ultra-precise and efficient fashion.
It is therefore desirable to provide a non-planar optical diffraction grating having grooves which include a plurality of sub-grooves that can be utilized in various optical imaging devices (e.g. spectrometers or hyperspectral imagers) which require an extremely sensitive diffraction grating, and that does not suffer from the above drawbacks experienced by diffraction gratings having only grooves (i.e. no sub-grooves). Additionally, while addressing these problems, the optical diffraction grating having grooves which include a plurality of sub-grooves of the present invention will simultaneously provide an optical diffraction grating having a superior ultra-precise non-planar surface shape desired in extremely sensitive and complex optical imaging devices.
These and other advantages of the present invention will become more fully apparent from the detailed description of the invention hereinbelow.